This will be a really fun project to build and enjoy Max-Machine! I’m planning a post on this but for you right now, see the following info. It should help you get started.
For profiles, racing sims are of either 40×40 and 40×80 profile or 45×45 and 45×90 profile. Generally, the 40 Series profiles (40×40 and 40×80) since there’s are more durable from a strength perspective.
Helpful tip: Draw out your design with free software downloads for CAD from companies like PARCO to calculate the weight and hardware needed.
My advice on fasteners and hardware— don’t cheap out. Use what the company has so that everything fits and travels in the t-slot channel properly.